The present invention relates to an ultrasound diagnostic apparatus and a method of producing an ultrasound image, and in particular, to an ultrasound diagnostic apparatus and a method of producing an ultrasound image for measuring an intima-media thickness of a blood vessel.
An ultrasound diagnostic apparatus which carries out ultrasound diagnosis using an ultrasound image has hitherto been put into practical use in the field of medicine. In general, this type of ultrasound diagnostic apparatus has an ultrasound probe embedded with a transducer array, and a diagnostic apparatus body connected to the ultrasound probe. The ultrasound diagnostic apparatus transmits an ultrasonic wave from the ultrasound probe toward a subject, receives an ultrasonic echo from the subject by the ultrasound probe, and electrically processes the reception signal by the diagnostic apparatus body to produce an ultrasound image.
In the ultrasound diagnostic apparatus, various kinds of information which represent the state of a disease can be obtained based on the reception signal obtained by receiving the ultrasonic echo from the subject. For example, in order to obtain information regarding a circulatory system disease, such as arteriosclerosis or cerebral infarction, transmission and reception of an ultrasonic wave is carried out with respect to a blood vessel, and the intima-media thickness (IMT) or the like of a vascular wall is calculated based on the obtained reception signal. The value of the intima-media thickness changes along with the progress of arteriosclerosis, and the state of the circulatory system disease can be estimated by monitoring the value.
However, the vascular wall has a small thickness and further, noise is mixed in the reception signal due to the influence of a pulsation accompanying heartbeat, or the like, and therefore, it is difficult to accurately calculate the intima-media thickness of the vascular wall in the ultrasound image.
Accordingly, as a technique which accurately measures the intima-media thickness of a vascular wall in an ultrasound image, for example, as disclosed in JP 2008-168016 A (Patent Document 1), an ultrasound diagnostic apparatus which detects the boundary of an intima-media complex (vascular wall) based on the slope and the amount of change in the intensity distribution of sound ray signals to calculate an intima-media thickness has been suggested.
T. Gustavsson, Q. Liang, I. Wendelhag, and J. Wikistrand, “A dynamic programming procedure for automated ultrasonic measurement of the carotid artery,” in Proc IEEE Computers Cardiology 1994, pp. 297-300 (Non-Patent Document 1) discloses a method which detects the boundary position of an IMT using a dynamic programming method based on an evaluation function including the intensity of a sound ray signal and the difference in the boundary position between adjacent sound rays.
WO 2011/099102 A1 (Patent Document 2) discloses an ultrasound diagnostic apparatus including a pattern similarity calculating unit which calculates pattern similarity based on a template representing a reference pattern of a vascular wall prepared in advance, and a boundary continuity calculating unit which calculates boundary continuity based on the difference in intensity value among a plurality of adjacent sound ray signals. In the ultrasound diagnostic apparatus, a boundary portion is determined based on a boundary evaluation value.
In the ultrasound diagnostic apparatus disclosed in Patent Document 1, a threshold value in evaluating the amount of change in luminance between the sound ray signals is set, and when the boundary of the intima-media complex is not detected, the threshold value is gradually decreased and the search range is gradually narrowed down, thereby distinguishing between the intima-media complex and noise and measuring the intima-media thickness of the blood vessel.
However, in the ultrasound diagnostic apparatus of Patent Document 1, when some of the sound ray signals drops out and the intensity thereof is degraded significantly, there is a high possibility that a vascular posterior wall intima-lumen boundary is erroneously detected as a lumen side, and it is not possible to measure the intima-media thickness with high precision.
In addition, in Non-Patent Document 1, since continuity of adjacent sound rays depends on only difference information of the boundary position, there is a problem in that a point where a signal is weak represents the same position as an adjacent signal ray, and a region, such as plaque, where intensity largely changes cannot be tracked.
Moreover, in Patent Document 2, continuity of adjacent sound rays is determined based on the difference in intensity value, and accordingly, as in Non-Patent Document 1, there is a problem in that the ultrasound diagnostic apparatus is susceptible to a portion where there is change from a point at which a signal is strong to a point where a signal is weak, and in particular, is susceptible to a case where noise produced in a vascular posterior wall and noise produced in a vascular lumen are close to each other in position (fogging) and strong. In addition, pattern matching using the reference pattern can track a normal region, but is susceptible to hyperplasia, plaque, or the like.